1. Field of the Invention
The invention relates to the field of optical inspection systems for assessing the quality of welds. In particular, the invention concerns an automated part handling and image processing system responsive to sharply localized variations in reflectance, for selecting, rejecting and/or monitoring the quality of girth welds on nuclear fuel tubes.
2. Prior Art
Nuclear fuel is packaged in long narrow stacks of enriched uranium pellets carried in zirconium alloy tubes. The tubes are plugged at the ends and sealed, and in addition to the pellets, contain an inert gas. A number of such tubes are mounted parallel to one another in a fuel assembly having supporting grids spaced along the tubes, with receptacles holding the tubes parallel to one another and at a space sufficient to admit control rods that are movable into spaces between the tubes for damping nuclear flux.
Zirconium is similar to aluminum in that it is a malleable metal that develops an oxide film or coating on surfaces exposed to oxygen, the coating generally forming a barrier that protects the metal underneath. When subjected to heating in oxygen or in water (for example when used in a reactor for generating heat), the oxide coating thickens and the tube becomes black, the rate of oxidation being related to the temperature and time of heating and the availability of oxygen. Zirconium alloy (e.g., Zircalloy) is a preferred material for fuel rods because it presents a low cross section to nuclear particles such as neutrons and therefore does not in turn become a strong source of nuclear radiation when it is irradiated.
Oxidation of the fuel rod tubes is advantageous because a thick oxide coating protects the tubes, giving them a harder surface. Reactor coolant pumps and convection currents in an operating reactor produce a powerful and turbulent flow of coolant in pressurized water reactors and boiling water reactors. The flow can carry along pieces of metal and the like, which can impact against the fuel rod tubes. It is important to avoid a breach in the walls of the fuel rod tubes, which can lead to release of radioactive material into the coolant. Oxidized tubes are harder and less subject to fretting damage from debris carried along in the reactor coolant.
The tubes are particularly vulnerable to fretting damage when they are new and relatively unoxidized. The tubes are also vulnerable on their surfaces immediately adjacent to the supporting grids of the fuel assembly on the upstream side relative to coolant flow. Loose debris in the coolant can be caught by the supporting grids and fretted against the tubes in this area. Such fretting damage to the tubes has been found to occur most often on the upstream side of the foremost supporting grid of the fuel assembly, which is the lowermost grid due to the upward flow path of the coolant, namely at the lower end of the fuel rod tube.
To combat the danger of fretting damage at the upstream end of the tubes, it has been proposed to treat the tubes preliminarily at their upstream ends, for example along the lowermost six or eight inches (15-20 cm), to protect the tubes from fretting damage. U.S. Pat. No. 5,171,520 --Bryan et al proposes to coat the tubes with zircon, a hard refractory material (ZrSiO.sub.4) also known as zirconium silicate. U.S. Pat. No. 5,265,137 --Busch proposes to treat at least the ends of tubes by heating them with one or more of carbon, nitrogen and oxygen to form a protective layer.
During production, an end plug is welded to the fuel rod tube, which can be an automated process as disclosed in U.S. Pat. 4,857,260 --Schoenig, Jr. et al. The fuel pellets are loaded, together with an inert gas, and the opposite end plug is welded in place. The attachment of the end plugs to the tube is made along a circumferential line and the weld is termed a girth weld. Typically, the fuel rods are visually inspected for quality, including for the integrity of the girth welds and other aspects such as dimensions.
Insofar as the tubes or the ends of the tubes are treated for surface hardness by oxidizing and thereby blackening the tubes, or by applying a protective coating, the girth welds are obscured. It is possible to inspect the tube ends using X-rays, which are not sensitive to the appearance of the surface. Such a technique is disclosed in U.S. Pat. No. 4,957,691 --Brashier et al. For optimal visual inspection, it is appropriate to inspect the tubes before such treatments, and preferably immediately before, so that the potential for handling damage between inspection and treatment is minimal.
It would be advantageous to automate the production of fuel rods as much as practicable, including the inspection of the tubes. Automated handling and processing steps, however, are generally inconsistent with visual inspection for dimensions, defects, weld quality and the like.
The present invention seeks to provide an automated technique for optical inspection of girth welds which can be accomplished with minimal handling steps using a compact apparatus that rotates the tube on its longitudinal axis while pixel data is collected by a line scan camera triggered by a shaft angle encoder coupled to the tube rotation drive. Each scan line is digitized, obtaining a line of pixel data over a longitudinal length encompassing the girth weld, and the scans are collected at equally spaced angular positions around the tube. The invention further seeks to analyze reflectance data on the tube in the area of the weld, in a manner that amplifies the system's reaction to local flaws that are equal to or larger than a predetermined minimum, by determining the average value of a number of pixels and counting the number of adjacent pixels, in two mutually perpendicular directions, which exceed the average reflectance value by a predetermined proportion (i.e., profiling defects parallel to the axis of the tube and along a circumference). The number of pixels and their pitch or spacing corresponds to the minimum flaw size, profiled in the X and Y directions in the collected matrix of pixel data. The image is thereby analyzed for detection missing or gapped welds, automatically determining the quality of the plug/tube seam.